Various devices can be associated with an internal combustion engine that powers a motor vehicle to brake the engine by itself. Such engine brakes can be useful in larger vehicles like highway trucks. A known technique for retarding an internal combustion comprises augmenting engine back-pressure. One way of doing this comprises restricting the exhaust gas flow from the engine. In a conventional camshaft engine, a valve that is disposed in the exhaust system, sometimes called an exhaust brake, can be operated to restrict the exhaust gas flow. In an engine that has variable valve actuation, the individual cylinder exhaust valves may be actuated in a manner that creates the desired restriction.
It is known the use hydraulic control fluid for operating an engine brake. When the brake is to be applied (activated), fluid under pressure is delivered to an actuator for the brake to operate the brake. When the brake is to be released (de-activated), the fluid is dumped from the actuator to relieve the applied pressure and allow the brake to release.
Certain diesel engines have fuel injection systems that utilize hydraulic fluid under pressure to force fuel into engine combustion chambers. The hydraulic fluid is supplied to a respective fuel injector at each engine cylinder. When a valve mechanism of a fuel injector is operated by an electric signal from an engine control system to inject fuel into the respective cylinder, the hydraulic fluid is allowed to act on a piston in the fuel injector to force a charge of fuel into the respective combustion chamber.
A running engine experiences a transition in operation when an engine brake is released and the engine returns to delivering positive power for propelling a vehicle. If fueling is not suitably controlled during the transition, the transition may not be as smooth as desired. A rough transition is evidenced by engine misfire and the consequent generation of excess smoke in the engine exhaust.
It has been observed that a contributing factor to engine roughness during such transitions is the rate at which the engine brake releases. If the hydraulic fluid that is activating the brake is not dumped sufficiently fast from the actuator, cylinder misfires and extra exhaust smoke may result. For example, delayed release of a brake acting on engine exhaust valves can cause them to stay open longer than desirable, potentially causing misfires and extra smoke in the exhaust. Hence it is generally desirable to dump the hydraulic fluid as rapidly as possible so that engine braking can promptly end in anticipation of a return to positive power delivery. But during dumping, fueling must be controlled in a way that can accommodate the more rapid brake de-activation.
Commonly owned U.S. Pat. No. 6,807,938 of the inventors discloses a strategy for limiting fueling after de-activation of an engine retarder that had previously been activated to brake an engine. While limiting and/or delaying fueling during the transition can provide some improvement, it is believed that further improvement would be desirable during such transitions.